1. Field of the Invention
This invention relates to a moving boundary type time-temperature history device for measuring the shelf-life of perishable products. The device is constructed of an activating tape, containing an activator composition in a matrix, an indicating tape, containing an indicating composition in a matrix, adhered together forming a wedge-shaped composite matrix by means of adhesives, preferably pressure sensitive, in which the activator composition diffuses through the wedge-shaped composite matrix to contact the indicator composition producing a moving observable color change along the length of the device.
2. Brief Description of the Prior Art
Perishable products have measurable shelf-lives, which are usually expressed within specified limits as the time left for available end use. By the term "perishable products" is meant to include perishable foods having a measurable shelf life such as fresh, refrigerated, and frozen, vegetables, fruits, meats, fish, poultry, dairy products, bakery products, juices, pre-cooked foods, soft and alcoholic beverages, and also including nonfood, items having extended shelf lives ranging from a few hours to some years including pharmaceuticals, vaccines, sera, blood, blood plasma, cosmetics, reactive chemical compounds, biochemical products, batteries, x-ray film and photographic films. For example, FIG. 2 shows the shelf lives of various perishable products plotted in natural logarithm of shelf life in months against reciprocal of absolute temperature resulting in straight line graphs. For example from the graph, it can be seen that Eact's for corn cereal and strawberry are 13.5 and 44.0 kcal/mole. The shelf lives for frozen asparagus, poultry and strawberry at 18.degree. C. are 180, 360 and 630 days.
A quantitative view of hypothetical product decay rates is seen in FIG. 1. In general, by plotting the natural logarithm of the shelf life or decay rate of product versus the reciprocal of the absolute temperature (1/T in degrees Kelvin) a straight line is generally obtained defining the temperature sensitivity characteristics of product decay. The slope of the line is referred to as Eact.,(or E) the energy of activation of the product decay and the ordinate intercept (Y intercept) is a constant for the decay process. The same straight line graph can be used to illustrate the rate characteristics for the color change of a device.
The well-known Arrhenius equation describes the characteristics of the above-described straight line produced in FIG. 1: EQU ln k=A-Eact/RT
where,
ln k=natural logarithm of the reaction rate PA1 A=pre-exponent constant, also referred to as the rate constant PA1 Eact=activation energy PA1 R=universal gas constant PA1 A=absolute temperature in degrees Kelvin.
FIG. 1 also illustrates difficulties involved in the use of a time-temperature history indicator device which undergoes an observable color change to monitor the shelf life of the perishable product at different temperatures. If the perishable product decay is line B and the indicator device is line A, then at Ti, it is seen that the product decay and rate of color change of the indicator are identical and thus the indicator will accurately monitor the shelf life of the perishable product at this temperature.
However, a problem arises if the perishable is stored at the higher temperature Th. In this case, the indicator A will undergo color change prior to expiration of the usable shelf life of the product thus causing the product to be prematurely discarded. Conversely, if the perishable is stored at lower temperature Tl, the shelf life of the product will expire prior to the termination of the color change of the indicator, a potentially dangerous situation for those perishables which for example, can become contaminated with harmful bacteria, e.g. botulism. As is seen in FIG. 1, the problem with using the indicator A, to monitor the shelf life of perishable B, is that the slopes of the lines. i.e. energies of activation, are not identical.
Further, assuming that line C is the indicator and line B the perishable, it is seen that the two lines are parallel and have identical Eact's. However, since the rate constant (Y intercept) for C is much lower than that for B, monitoring the shelf of the perishable B at any temperature, will result in the indicator changing color at a significantly prior time to the end of the shelf life, resulting in a premature rejection of the product. The problem here is that even though the Eact for product decay and indicator color change are identical, the rate constants for the two processes are different and thus lead to erroneous shelf life monitoring.
Thus, in order to be an effective indicator for monitoring the shelf life of perishable product, the characteristics of the indicator versus the product must be such that preferably both the Eact and rate constant of the color change are substantially identical to those of product decay of the perishable product, i.e. they both exhibit substantially identical Arrhenius graphs.
Further, it is highly desirable as in the case of perishables having a relatively short shelf-life, i.e. milk, fish, eggs, and ice cream, to be continuously aware of the remaining shelf-life of a perishable product so that contingencies can be made for their disposal, rather than be confronted with an unexpected abrupt color change signifying termination. It is also desired that the device be in a form which is easily manufactured and readily applied and adhered to the perishable, such as a "tape" device. Thus, what is highly desired in the art is a monitoring device preferably affixed to a perishable as a "tape", which continuously and accurately illustrates the remaining shelf life of the perishable.
A large number of devices have been reported in the patent literature for monitoring thermal degradation of perishables.
For example, several patents describe time-temperature monitoring (TTM) devices based on diffusion of liquids, vapors or gases through a barrier film. For example, in U.S. Pat. Nos. 4,195,056 and 4,195,058, G. N. Patel describes a device based on diffusion of vapor through a barrier film to introduce color change in the indicator on the other side of the barrier. In order to prevent escape of the vapor, the device is sealed in a plastic container and requires a solvent reservoir.
A somewhat similar device is described by Giezen et al in U.S. Pat. No. 4,154,107. The device utilizes an activator acid in a pressure sensitive adhesive which migrates to contact an organic dye producing an aqueous-mediated color change. The preferred device also requires an absorbant paper element to contain the indicator and a wetting agent to retain water to introduce color change. The activating/indicating components used in the device are water soluble and hence the performance of the device is adversely affected by moisture and humidity. In order to protect the device, an enveloping plastic film is employed.
Kydonieus et al in U.S. Pat. No. 4,212,153 describe a device for monitoring product shelf life in which a dye preferably migrates from a lower plastisol layer to an upper indicator layer, being preferably polyvinylchloride. The device can also utilize a barrier film to introduce an induction period to color change, but doesn't describe a tape device, or the separate use of an activator and indicator agent.
Bradley et al in U.S. Pat. No. 4,292,916 describe a tape device for monitoring shelf life which involves the migration of a dye from a carrier layer to a transfer layer to highlight a message. However, the device preferably uses a porous barrier such as cheese cloth, requires a protective cover, does not describe the use of pressure sensitive adhesives to bond the different layers of the device together and requires an impervious covering layer for the entire indicator device.
U.S. Pat. No. 3,520,124 to Myers describes a device to indicate a predetermined time interval based on two or more materials which react, either chemically or physically over a predetermined period to produce a termination signal. The reacting materials are carried on a base member and are separated by a barrier preventing contact. Upon elimination of the barrier, a commencement signal is produced indicating the time reaction is underway. However, there is not described a tape device or the use of pressure sensitive adhesives for bonding the layers of the device together.
Other patents in the art include: U.S. Pat. No. 3,677,088; U.S. Pat. No. 3,967,579; U.S. Pat. No. 3,360,338; U.S. Pat. No. 4,057,029; U.S. Pat. No. 3,065,083; U.S. Pat. No. 4,188,437; U.S. Pat. No. 2,889,799; U.S. Pat. No. 3,078,182; U.S. Pat. No. 3,311,084; U.S. Pat. No. 3,386,807; U.S. Pat. No. 4,154,107.
However, the above devices are "stationary" devices since the appearance of color is usually generated over the entire indicating surface simultaneously and do not provide a continuous observable account of available shelf life.
Also described in the art are "moving boundary" devices which operate on the principle of a moving band of produced color so that the continuously elapsed time of the monitored period can be visually observed.
For example U.S. Pat. No. 4,382,700 to Youngren describes an indicator which in a preferred embodiment includes a material such as mineral jelly, which is in contact with a wick, such as a wedged paper strip, such that the mineral jelly diffuses into the paper in accordance with changes in ambient temperature over a period of rime. The wedge shape of the paper strip increases the time for color production transversely along the strip thus creating a moving boundary indicative of elapsed time.
Other moving boundary devices which are also described in the art include: U.S. Pat. No. 4,196,057 and U.S. Pat. No. 4,196,055 both to G. N. Patel; U.S. Pat. No. 4,432,656; U.S. Pat. No. 2,614,430; U.S. Pat. No. 3,479,877; U.S. Pat. No. 3,414,415; U.S. Pat. No. 3,942,467; U.S. Pat. No. 3,954,011; U.S. Pat. No. 3,981,683; U.S. Pat. No. 4,057,029; U.S. Pat. No. 4,163,427; U.S. Pat. No. 4,280,441; U.S. Pat. No. 4,410,493; U.S. Pat. No. 4,509,449.
However, the above referred-to devices generally use liquids or vapors as the activator in the color-indicating systems rather than a solid water-impermeable material which is required in a tape. Further the references do not specifically teach one skilled in the art how to design a particular tape device in order to match the product decay characteristics particularly with respect to the Eact and the rate constant to accurately monitor the perishable product over a range of storage temperature conditions.